1. Field of the Invention
The present invention relates to an improvement of an observation apparatus such as an operation microscope apparatus or a slit lamp microscope apparatus, and more particularly to an observation apparatus capable of removing astigmatism and chromatic aberration which occur when a state of an eye is observed.
2. Description of the Related Art
Up to now, there has been widely known an observation apparatus such as an operation microscope apparatus used for observing an eye which undergoes an operation (hereinafter referred to as “an eye to be operated”) (see JP 2003-062003A, specification paragraphs [0029] to [0032], FIGS. 4, 5, and 9) or a slit lamp microscope apparatus used for observing the anterior ocular segment and fundus portion of an eye to be examined (see JP 2001-037726 A, specification paragraphs [0017] to [0020], FIG. 1).
Hereinafter, such an observation apparatus, in particular, an operation microscope apparatus will be described. An operation microscope apparatus having an external structure as shown in FIG. 1 has been known as an example. The operation microscope apparatus shown in FIG. 1 includes a support post 1 for supporting the apparatus onto a floor surface, a support arm 2 for supporting an operation microscope main body, and bracket 3 for operation microscope main body attachment, which is bonded to a tip portion of the support arm 2.
The support arm 2 is composed of an L-shaped arm 4 and a pivot arm 5. The L-shaped arm 4 is bonded to an upper end of the support post 1 to be pivotable in the lateral direction. The pivot arm 5 is urged upward by elastic action of a spring stored in the arm.
An arm 6 supported to be pivotable in the lateral direction is provided to a tip portion of the pivot arm 5 to face downward. The bracket 3 is bonded to a lower end portion of the arm 6. An operation microscope 10 composing a main body of the operation microscope apparatus is attached to the bracket 3.
The operation microscope 10 is mainly composed of a lens barrel 11 that stores various optical systems. The lens barrel 11 is provided with an eyepiece lens barrel 11′ for observation of the eye to be operated as an observation object by an operator.
For example, an illumination optical system 12 and an observation optical system 13 as shown in FIG. 2 are placed in the lens barrel 11 of the operation microscope 10. The illumination optical system 12 used for illuminating an eye to be operated E includes an illumination light source 14, a condensing lens 15, an illumination field stop 16, a collimator lens 17, a prism 18, and an objective lens 19, which are disposed in this order. Note that reference numeral 18b denotes a reflective surface of the prism 18. Illumination light emitted from the illumination light source 14 is reflected on the reflective surface 18b of the prism 18 through the condensing lens 15, the illumination field stop 16, and the collimator lens 17. The reflected illumination light is guided to the eye to be operated E by the objective lens 19 to illuminate a pupil Ea, an iris Eb, and a cornea Ec.
The observation optical system 13 is an optical system for receiving observation light for observing the eye to be operated E, which is illuminated by the illumination optical system 12. As shown in FIG. 3, the observation optical system 13 is composed of a left observation optical system 13a and a right observation optical system 13b. 
The left observation optical system 13a includes the objective lens 19, a variable lens system (zoom lens system) 20 composed of lenses 20a, 20b, and 20c, a beam splitter 21, an imaging lens 22, an image erecting prism 23, an interpupillary adjustment prism 24, a field stop 25, and an eyepiece 26, which are disposed in this order. Note that reference numeral 2a1 denotes an entrance pupil and 26a denotes an eye point.
Similarly, the right observation optical system 13b includes the objective lens 19, a variable lens system (zoom lens system) 30 composed of lenses 30a, 30b, and 30c, a beam splitter 31, an imaging lens 32, an image erecting prism 33, an interpupillary adjustment prism 34, a field stop 35, and an eyepiece 36, which are disposed in this order. Note that reference numeral 2b1 denotes an entrance pupil and 36a denotes an eye point.
Reflection light (observation light) on the eye to be operated E, which is illuminated by the illumination optical system 12, passes through the objective lens 19. Then, the observation light is guided to the right and left eyes of the operator through respective optical elements of the left and right observation optical systems 13a and 13b. A part of the observation light is reflected by the beam splitters 21 and 31 to be guided to an auxiliary observation optical system 40 and a TV image pickup system 50.
The auxiliary observation optical system 40 is an optical system used for observation of the eye to be operated E by an operator's assistant and includes an imaging lens 41, a reflecting mirror 42, and an eyepiece 43. The TV image pickup system 50 is an optical system for taking an image of the eye to be operated E and includes an imaging lens 51, a reflecting mirror 52, and a TV camera 53. The TV camera 53 is provided with a CCD image pickup element 53a serving as an image receiving unit.
FIG. 4 is a schematic view in the case where the observation optical system 13 shown in FIG. 3 is viewed from the above. In FIG. 4, an optical axis of the objective lens 19 is indicated as “O” and optical axes (observation optical axes) of the left and right observation optical systems 13a and 13b are indicated as O1 and O2. A surface 18a of the prism 18 becomes an exit pupil of the illumination optical system 12 and is disposed near observation optical paths 2a2 and 2b2 of the left and right observation optical systems 13a and 13b. 
According to such an operation microscope apparatus, it is possible to observe the anterior ocular segment of the eye to be operated E. However, it is impossible to observe the surroundings of a fundus Er (fundus surroundings) of the eye to be operated E without changing the structure of the apparatus. In order to facilitate the observation of the fundus surroundings, utilized in many situations is a method of illuminating fundus surroundings Er′ with a state in which an optical member 60 such as a prism having a predetermined apex angle (for example, 45 degrees) as shown in FIG. 5 is put to the cornea Ec of the eye to be operated E so as to reduce refracting power of the cornea Ec and to refract illumination light. An intraocular observation contact lens as disclosed in JP 05-023304 A (specification paragraphs [0016] to [0027], FIG. 1) is also used.
When the optical member 60 is put to the cornea Ec, light beams parallel to the optical axis “O” of the objective lens 19 and an illumination optical axis O′ of the illumination optical system 12 (see FIGS. 2 and 4) are refracted and, in addition, light beams parallel to the observation optical axes O1 and O2 of the left and right observation optical systems 13a and 13b are refracted. Therefore, it is possible to observe the fundus surroundings Er′. When, for example, a prism having one of various apex angles θ is used as the optical member 60 as appropriate, an observation region of the fundus surroundings Er′ can be changed as appropriate.
The number of types of the recent operation microscope apparatus as described in JP 2003-062003 A has been increasing, each of which includes an optical member (which is called a front lens) provided so as to be insertable between the eye to be operated and the objective lens, thereby allowing an operator to perform an operation with both hands.
When the optical member 60 having deflective action such as the prism or the contact lens is put to the eye to be operated E to observe the fundus surroundings Er′, astigmatism and chromatic aberration are caused by optical refractive and dispersive action. That is, with respect to a point image on the fundus surroundings Er′, when the optical member 60 is put to the eye to be operated E, the astigmatism occurs in which the point image to be observed is changed from a state of a longitudinal ellipse to a state of a transverse ellipse through a state of a minimum circle with changing a focusing state of the point image from the front side of a focusing position to the rear side thereof, thereby distorting an observation image. Therefore, the sharpness of the observation image deteriorates.
Even with respect to the chromatic aberration, when the optical member 60 is put to the eye to be operated E, the chromatic aberration occurs even at the focusing position in a direction of the apex angle of the optical member 60. When chromatic aberration is occurring, even if the astigmatism is removed, the chromatic aberration remains, so that the observation image appears to separate for each color. Therefore, the sharpness of the observation image deteriorates.
When the apex angle θ of the optical member 60 reduces or when a refraction index of a material composing the optical member 60 increases, the astigmatism and the chromatic aberration become more significant. Therefore, when the fundus surroundings further apart from the fundus Er are observed, the sharpness of the observation image further deteriorates.
The astigmatism and the chromatic aberration occur even in an eyeball optical system of the eye to be operated E. In particular, when an eye to be operated into which an intraocular lens (IOL) used for the treatment of cataract and the like is inserted is observed, the influences of the astigmatism and the chromatic aberration become larger.
The amount of astigmatism and the amount of chromatic aberration which are caused by the optical member 60 and the eyeball optical system of the eye to be operated increase substantially proportional to an exit angle of an observation light flux passing through the optical member 60.
When an operation is performed using the operation microscope apparatus, the operator usually observes the eye to be operated E over the head of a patient (located on a low side in FIG. 4). When the fundus surroundings Er′ located in the longitudinal direction viewed from the operator (height direction of the patient) are observed with respect to the fundus Er of the eye to be operated E, exit angles of right and left observation light fluxes passing through the optical member 60 (angles formed between the normal to an oblique surface of the optical member 60 and each of the observation optical axes O1 and O2) are equal to each other. However, when the fundus surroundings Er′ located in the lateral direction viewed from the operator are observed, the exit angles of the right and left observation light fluxes are greatly different from each other.
For example, as shown in FIG. 5, assume that the apex angle of the optical member (prism) 60 is given by θ and a convergent angle of the operation microscope (angle formed between the left and right observation optical axes O1 and O2) is given by β. When the fundus surrounding Er′ located on the left side viewed from the operator is observed, an exit angle αL of light parallel to the observation optical axis O1 of the left observation optical system 13a becomes (90°−θ+β/2) and an exit angle αR of light parallel to the observation optical axis O2 of the right observation optical system 13b becomes (90°−θ−β/2). Therefore, in such a case, the influences of astigmatism and chromatic aberration on an image of the fundus surrounding Er′ observed by the left eye of the operator are larger than those on an image of the fundus surrounding Er′ observed by the right eye of the operator. Similarly, when the fundus surrounding Er′ located on the right side viewed from the operator is observed, the influences of astigmatism and chromatic aberration on an image of the fundus surrounding Er′ observed by the right eye of the operator are larger than those on an image of the fundus surrounding Er′ observed by the left eye of the operator.
As described above, when the aberration amounts of the images observed by the right and left eyes are different from each other, it is hard for the operator to normally stereoscopically view the image of the fundus surrounding Er′.
The above-mentioned problem related to the astigmatism affects the case where coagulation treatment is performed on the fundus surroundings using laser light with the optical member such as the prism having the deflective action put to the cornea. Even when the above-mentioned front lens is put between the eye to be operated and the objective lens, the problems related to the astigmatism and the chromatic aberration occur.